This invention is directed to isotropic carbon fibers and to methods for making the same from liquid hydrocarbon distillates.
Carbon fibers have been known since 1880 when Thomas Edison first used a carbon fiber as an incandescent lamp filament. Presently carbon fibers are being incorporated into other materials such as, for example, metals, polymers, carbon, graphite, ceramics and the like to make carbon fiber reinforced composites that have improved mechanical, thermal and electrical properties. Such composites are used particularly in the present aerospace industry as the thermally stable and ablative material.
Heretofore, two basically different filaments have been made from carbon. One type is known as a carbon fiber and is made by baking carbonaceous materials at relatively low temperature (about 1000.degree. C) and the second type is known as graphite fiber and is made by heating carbon fibers at relatively high temperatures (about 2500.degree. C or higher.) The graphite fibers are stronger than carbon fibers. Graphite fibers have a tensile strength of about 80,000-160,000 psi and a modulus of elasticity of about 50.0-100.0 .times. 10.sup.6 psi and carbon fibers have a tensile strength of about 80,000-160,000 psi and a modulus of elasticity of about 6.0 - 8.0 .times. 10.sup.6 psi.
Isotropic carbon as used herein refers to elemental carbon having a physical structure such that it has the same physical and chemical properties in any given spatial direction. In appearance, isotropic carbon is a hard, glass like infusible shiny black substance. Whether a particular sample of carbon is isotropic carbon or anisotropic carbon (i.e. having a physical structure such that it exhibits different physical and chemical properties according to its structural orientation) is determined by the miscroscopic observation of a sample of the carbon in a metallographic microscope. Typically, a sample of the carbon is mounted in an epoxy-type resin mount and polished to a mirror smooth surface finish. The polished surface is observed under a conventional metallographic microscope in reflected polarized light with cross Nicols. If the sample is isotropic carbon there will be no change in the intensity of reflected polarized light as the sample is rotated; however, the intensity of the reflected polarized light will change upon rotation if the sample is anisotropic carbon.
Another unique characteristic of isotropic carbon is that it is nongraphitizing. In general, carbons can be classified into graphitizing and nongraphitizing carbons. Examples of the former carbon are pitch and petroleum cokes. Examples of non-graphitizing carbons are those derived from polyvinylidene chloride, cellulose, sucrose and the like. For graphitizing carbons the term soft carbon is often used and for non-graphitizing the term hard carbon or turbostratic carbon is often used. Isotropic carbon can be partially graphitized only with difficulties. If isotropic carbon is subjected to temperatures of 2500.degree. C or higher graphite is only formed to a very limited extent.
Isotropic carbon is characterized further by being very resistant to attack by strong mineral acids including hydrofluoric acid. It also has a lower rate of oxidation than other forms of carbon. The internal friction of isotropic carbon is only a fraction (about 1/4) of other non-crystalline carbons.
Isotropic carbon is not new and is well repented in the literature. See, for instance, the article by T. Yamaguchi in Carbon, Vol. 1, 47-50 (1963) and the article by T. Tsuzuku and H. Kobayashi at Pge. 539 of the Proceedings of the Fifth Carbon Conference. In addition, see the article by Fitzer, Schoefer and Yomada in Carbon, Vol. 7, pp. 643-648 (1969.) Isotropic carbon is also the subject of several patents. See for example, British Pat. No. 1,182,455 which is directed to preparation of isotropic carbon from pitches reacted with ammonium sulphate. In addition, see Krellner, U.S. Pat. No. 3,284,371 which is directed to isotropic carbon for electrographitic brushes.
Heretofore the most common method for preparing isotropic carbon was by carbonizing highly crosslinked macromolecular structures which, in addition to carbon and hydrogen, contain oxygen, nitrogen or sulfur. Carbonization of such components form rigidly crosslinked aromatic planes which prevent further conversion to a graphite structure. Examples of some of these compounds are sucrose, cellulose, rayon, furfural phenolic resin, phenol formaldehyde resins, acetone furfural polymers, polyvinylchloride, polyvinylidenchloride and polyacrylonitirile.
Carbon fibers are also known in the art. For example, see the article by S. Otani found in Carbon, Vol. 3, pp. 31-38 (1965); Carbon Vol. 3, pp. 213; and Carbon Vol. 4, pp. 425-432 (1966). Several recent patents are also directed to carbon fibers. See Otani, U.S. Pat. Nos. 3,392,216 and 3,629,379. In addition, see Joo et al, U.S. Pat. No. 3,595,946 and Shea et al, U.S. Pat. No. 3,668,110. However, none of the fibers cited above are isotropic carbon fibers which were made from hydrocarbon distillates.
Therefore, it is the object of this invention to provide isotropic fibers and a method for making the same from liquid hydrocarbon distillates.